It is well known that inductively coupled electrodeless low pressure discharge lamps offer many advantages. A typical inductively coupled discharge lamp comprises a lamp bulb which is sealed in a vacuum-tight manner and is filled with a metal vapor and a rare gas at a very low pressure. The inductor is energized by a high-frequency power supply (above 20 KHz) and thus provides a discharge in the space between the inductor and a fluorescent layer covering the internal surface of the lamp bulb.
A problem occurring during the operation of a gas discharge lamp is that electromagnetic fields are produced outside the lamp which cause high frequency interference currents in the power supply lines. As a result, especially due to the magnetic component of the field, disturbances may occur in other electrical apparatuses (such as radio and TV receivers) connected to the supply lines. Therefore, reduction of electro-magnetic interference (EMI), and especially its magnetic component, is one of the most important issues for commercially viable inductive discharge lamps.
Attempts have been undertaken in the field to reduce a magnetic flux that is found outside the lamp envelope of inductively coupled discharge lamps.
For example, U.S. Pat. Nos. 4,245,179 and 4,254,363 describe inductive primary coil geometries intended to reduce the total magnetic flux from the discharge. However, these techniques are generally not very practical, and there is no readily-available data demonstrating their effectiveness in reducing external magnetic flux.
U.S. Pat. Nos. 4,645,967, 4,704,562, 4,727,294, 4,920,297 and 4,940,923 teach a set of conductive short circuited anti-interference rings 10, 11, 12 that are attached to the outside of the lamp envelope and surround the discharge vessel (best shown in FIG. 1). When a discharge is inductively excited, these rings 10, 11 and 12 create a current which induces a magnetic flux in a direction opposite to the primary flux that neutralizes some of the magnetic flux of the primary induction coil. Disadvantageously, this technique is not very effective and is found to reduce the magnetic flux emitted from the discharge by only about 1.8 to 2.0 decibels (dB) per ring. More effective techniques for reducing the magnetic component of electro-magnetic field produced by the discharge lamp, would be highly desirable in the field.